Fun with Iterators - UnionIterator

The problem
You want to create an iterator for a union of iterators. For example, you have 5 iterators and you want to create an iterator so that it represents elements in all 5 of them in linear fashion.

Solution
(For brevity and simplicity, the UnionIterator is a readonly iterator.)

 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Iterator;
 import java.util.List;
 import java.util.NoSuchElementException;
 
 public class ReadonlyUnionIterator<E> implements Iterator<E> {
     
     private final Iterator<? extends Iterator<E>> parts;
     
     private Iterator<E> current = null;
     
     public ReadonlyUnionIterator(final Iterator<? extends Iterator<E>> parts) {
         this.parts = parts;
         alignForwardCurrent();
     }
     
     public static <E>ReadonlyUnionIterator<E> fromIterables(
             final Iterable<? extends Iterable<E>> iterableParts) {
         final List<Iterator<E>> itlist = new ArrayList<Iterator<E>>();
         for (final Iterable<E> each: iterableParts) {
             itlist.add(each.iterator());
         }
         return new ReadonlyUnionIterator<E>(itlist.iterator());
     }
     
     public static <E>ReadonlyUnionIterator<E> fromIterables(
             final Iterable<E>... parts) {
         return fromIterables(Arrays.asList(parts));
     }
     
     /**
      * Aligns 'current' by advancing to a non-empty element.
      * @return true if current has next element, false otherwise
      */
     private synchronized boolean alignForwardCurrent() {
         while (parts.hasNext()) {  // parts has next, but they may be empty!
             current = parts.next();
             if (current.hasNext()) { return true; }
         }
         current = null;
         return false;
     }
     
     public synchronized boolean hasNext() {
         // NULL is end-of-union marker for 'current'
         if (current == null)   { return false; }
         if (current.hasNext()) { return true; }
         // control reached here means current does not have next
         return alignForwardCurrent();
     }
     
     public synchronized E next() {
         if (hasNext()) { return current.next(); }
         throw new NoSuchElementException("No more element available");
     }
     
     public void remove() {
         throw new UnsupportedOperationException(
                 "Readonly iterator -- remove() is not supported");
     }
     
 }